The adsorptive separation of gas mixtures by means of pressure swing adsorption was developed for a very great variety of separation processes. All separation processes are based on the fact that the percentage of gas in the gas mixture that has a higher affinity for the adsorbent is fixed in an adsorption step on the surface of the adsorbent and the less strongly adsorbed component can be removed from the adsorber packed with the adsorbent. The desorption of the adsorbed phase is achieved by reducing the pressure after the adsorption step and by flushing the adsorbent with part of the enriched gas.
A device for enriching air with oxygen has become known from EP 135 936 A2. In the prior-art device, the air drawn in by a compressor is first cooled in a cooler to a temperature close to the ambient temperature and water of condensation is separated in the process. The air then reaches an adsorption bed, which comprises a first layer for separating water vapor and carbon dioxide and a second layer consisting of an X type molecular sieve for the adsorption of nitrogen. To separate the nitrogen, the air that is under the working pressure is first admitted to the adsorption bed, and the product gas obtained is stored intermediately in a tank. The pressure is lowered to a mean value below the working pressure. The residual pressure left is lowered to ambient pressure level during the subsequent desorption of the water vapor and the nitrogen, and the flush gas is drawn off into the environment via the feed point of the adsorption bed. To support the phase of desorption, the adsorption bed may be additionally flushed with the product gas.
The drawback of the prior-art device is that the moisture cannot be removed completely from the first layer of the adsorption bed if the flush gas flow is too low. However, increasing the flush gas flow leads to a no longer acceptable loss of product gas and consequently to low efficiency of the plant.